1. Field of the Invention
The present invention relates generally to a method and system for reducing chirp in optical communication and for providing a low-chirp externally-modulated laser.
2. Related Art
During laser modulation, a minimum level of spectral broadening necessarily occurs. Chirp, however, is an excess of spectral broadening beyond the spectral width required for modulation. Because different wavelengths propagate at different speeds in a dispersive medium, such as an optical fiber, the presence of significant chirp causes greater pulse spreading for a modulated optical signal transmitted across a fiber optic network.
Thus, reducing chirp has long been desired in optical communication. Low-chirp modulated laser sources are especially necessary in long-haul, high bit rate optical fiber transmission systems where chirp-induced pulse distortion reduces performance and range.
Two general approaches are typically used to modulate laser light: direct modulation and external modulation. In direct modulation, a laser such as a laser diode, is directly modulated by an information signal to generate a modulated laser output. In external modulation, a modulator is used to modulate light from a laser source such as a laser diode. An information signal is then applied to the modulator rather than to the laser as in direct modulation.
Moreover, in external modulation, two different arrangements are used for packaging the external modulator with respect to the laser source. The arrangement chosen also affects the amount of chirp. In the first arrangement, the modulator and laser are disposed on separate, discrete substrates. In the second arrangement, the modulator and laser are fabricated as an integrated modulator substrate on a common chip.
Each of these laser modulation approaches involve chirp. Under a direct modulation approach, the amount of chirp can be proportional to changes in the log of the laser output power over time. Thus, high-speed direct modulation of a laser output can produce substantial chirp, especially when the laser is driven to create sharp laser pulses with abrupt rising and falling edges. See, for instance, the Bickers and Westbrook article which describes reducing laser chirp in direct modulation by smoothing modulation pulse transitions ("Reduction of Laser Chirp in 1.5 .mu.m DFB Lasers By Modulation Pulse Shaping," Electron. Letts. 21 (3):103-104 (31st Jan. 1985)) and the Olshansky and Fye article which describes reducing chirp in direct modulation by using a small current step in the leading edge of the drive pulse (Olshansky, R. et al., "Reduction of Dynamic Linewidth in Single-Frequency Semiconductor Lasers," Electron. Letts. 20 (22):928-929 (25th Oct. 1984)), both of which are incorporated herein by reference.
External modulation is favored in applications sensitive to chirp, such as long-distance optical communications, where the excessive spectral broadening in the emitted modulated light due to chirp leads to a greater pulse distortion during propagation and a reduction in overall performance. In external modulation, however, chirp can further arise from electrical and optical interactions between the laser and the modulator. Thus, in the first external modulation arrangement discussed above, chirp can be reduced by isolating the discrete modulator and laser, electrically and optically, from each other. For instance, decoupling capacitors can be used to block stray DC current between the laser and modulator. An optical isolator which only allows light to travel in one direction can be inserted in a fiber or space between the laser and modulator to prevent reflections of light back from the modulator to the laser. See, for example, Suzuki, M. et at., "2.4 Gbit/s 100 km Penalty-Free Conventional Fibre Transmission Experiments Using GainAsP Electroabsorption Modulator," Electron. Letts. 25(3): 192-193 (2nd Feb. 1989); Nazarathy, M. et al., "Progress in Externally Modulated AM CATV Transmission Systems," J. Lightwave Technol. 11 (1):82-86 (January 1993)); and U.S. Pat. No. 5,420,868, issued to Chraplyvy et at., all of which are incorporated herein by reference.
Fabricating the laser and modulator separately, however, is inefficient and costly compared to an integrated modulator arrangement. Separate fabrication increases the overall size and complexity of the device and requires at least additional coupling fiber or optics between the laser and modulator.
Integrated modulators according to the second external modulation arrangement avoid the problems and inefficiencies associated with fabricating the laser and modulator as separate, discrete components. The electrical and optical interactions between the laser and modulator in an integrated modulator, however, are complex. See, for instance, the computer modeling of an integrated laser and electro-absorptive modulator discussed in Marcuse, D. et at., "Time-Dependent Simulation of a Laster-Modulator Combination," IEEE J. of Quantum Electron. 30 (12):2743-2755 (December 1994), which is incorporated herein by reference.
The complex optical and electrical interactions between the laser and modulator in an integrated laser-modulator combination make the reduction of chirp even more essential for these devices to be practical for optical communications systems. Because the laser and modulator are disposed in close proximity to one another on the same chip in an integrated laser modulator, it is more difficult to electrically and optically isolate the laser and the modulator from each other. See, the discussion of electrical and optical interactions in Suzuki, M. et al., "Electrical and Optical Interactions Between Integrated InGaAsP/InP DFB Lasers and Electroabsorption Modulators," J. Lightwave Technol. 6(6):779-784 (June 1988), which is incorporated herein by reference. In this 1988 Suzuki et al. article, RF bypass condensers can be connected to the laser to reduce electrical coupling. Such condensers increase cost and reduce the available chip space. High-quality anti-reflection (AR) coatings are provided on the end face of the modulator to reduce back reflections, but this too increases cost and complexity.
With respect to optical interactions, at present it is costly, if not impossible, to integrate an optical isolator on the same substrate with a laser and a modulator. Thus, one of the most effective ways of reducing chirp arising from back-reflected light in an external modulation arrangement (that is, placing an optical isolator in between the laser and modulator, as shown in the above-cited 1989 Suzuki et al. article) is unavailable for an integrated modulator.
What is needed then are further ways of reducing chirp occurring in external laser modulation. Chirp needs to be reduced in both cases of external modulation, where the laser and modulator are configured as separate, discrete devices and where the laser and modulator form an integrated modulator. Especially in the case of an integrated modulator, those optical interactions between the modulator and laser which give rise to chirp need to be reduced.